This invention is an improvement to the invention of Scott et al U.S. Pat. No. 4,767,515, patented Aug. 30, 1988, entitled "SURFACE AREA GENERATION AND DROPLET SIZE CONTROL IN SOLVENT EXTRACTION SYSTEMS UTILIZING HIGH INTENSITY ELECTRIC FIELDS", the entire disclosure of which is hereby expressly incorporated by reference.
The present invention relates generally to phase contactor methods and systems, such as for solvent extraction, wherein an emulsion is created for high interfacial surface area and, more particularly, to techniques for controlling the position and stability of the emulsion for enhanced coalescence.
Commercial solvent extraction systems are limited by the mass transfer rates of one or more chemical species between a continuous phase and a dispersed phase. Parameters limiting the mass transfer rate include surface area, convection, diffusion through each of the two phases, reaction rate and differences in chemical activity of the species in the two phases. Diffusion and chemical activity are determined by the system in use. The reaction rate is chemical in nature. Convection and surface area are physical considerations.
Two major problems which limit the use of solvent extraction in industrial applications are the efficient creation and control of mass transfer surface area. This surface area is the surface area of the dispersed phase in contact with the continuous phase. In practice, interfacial mass transfer surface area is usually created by a form of mechanical agitation or mixing. Generally, this mechanical agitation creates small droplets with a relatively high ratio of surface area to unit volume, as well as convection past the droplets. However, such mixing requires bulk movement of the continuous phase, thus decreasing the efficiency of the process. Since such systems require an energy input into the bulk of each liquid phase, energy is inefficiently used. In addition, agitation may create emulsions in the mass transfer apparatus which prove difficult to characterize and which can prove difficult to control during phase disengagement. Droplets can be difficult to coalesce, which can result in a significant increase in residence time within the apparatus. Moreover, mechanical mixers can break down causing problems and delays.
The above-incorporated Scott et al U.S. Pat. No. 4,767,515 discloses a method and system wherein droplets of a dispersed (e.g. aqueous) phase are introduced to a counter-current flow of a continuous (e.g. organic liquid) phase and are shattered by a high intensity pulsating electric field. These shattered droplets form an emulsion comprising a plurality of much smaller daughter droplets, which have a greater combined surface area for interfacial mass transfer than the original droplets. The daughter droplets coalesce into larger droplets of the dispersed phase material, which larger droplets then separate out of the continuous phase material.
An aspect of the method and system of Scott et al U.S. Pat. No. 4,767,515 which can benefit from improvement is that there is a tendency for the emulsion created by the electric field to be carried farther than desirable in the direction of the flow of the continuous phase, decreasing opportunity for coalescence.